This invention relates, in general, to power plants, and in particular, this invention relates to a control system for a marine turbine power plant.
One prior art example of a marine turbine power plant is found in U.S. Pat. No. 3,405,676 to Hobbs et al issued Oct. 15, 1968. The control system described in that patent is primarily a speed control system in that a desired speed input (throttle control) is continuously compared with an actual speed input (tachometer feedback) to produce a speed error signal when the desired and actual speeds differ. The speed error signal is input into an integrator circuit (speed control amplifier), the output of which is input into valve operating servos for changing valve position to compensate for the speed error. The integrator circuit output is continuous until the speed error is zero. Also shown in the Hobbs patent is a means for limiting the integrator output signal so that it does not exceed a voltage limit which would cause a boiler malfunction.
One object of the present invention is to eliminate the necessity of a speed feedback input under all operating conditions. It has been found that speed feedback at high rpm operating conditions is unnecessary for adequate ship control. Moreover, speed feedback at high rpm's can cause undesirable and unnecessary valve travel excursions in a speed control system because the system requires the actual speed to equal the desired speed even when such a requirement is not otherwise necessary for ship control.
Another object of the present invention is to provide a means for proportionately closing turbine valves to compensate for a boiler malfunction, an overspeed malfunction or a vibration malfunction. This is distinguished from a system which limits the opening of turbine valves in order to avoid causing a turbine trip.
Another object of the present invention is to select a rate for closing and reopening turbine valves in accordance with the aforementioned turbine malfunction proportional control and other system requirements.
In accordance with the aforementioned objects of the invention, the present invention is a marine turbine control system which is primarily a valve position control system having a selectively input speed error modification, a malfunction proportional control modification and a rate limiter modification. A throttle lever input is a valve position demand signal which is modified by a function generator to provide a valve position command linearly proportional to the valve position demand. Hence, valve position may be controlled without speed feedback.
As was previously alluded to, sometimes it is desirable to incorporate speed feedback into the turbine control system. For this reason, a speed feedback control loop is incorporated into the valve position control as a valve position command trimming signal. This is distinguished from the prior art wherein the speed error signal is the valve control signal.
An additional modification to the valve command signal may be a malfunction proportional control signal which overrides the valve position command signal to close the turbine valves upon the occurrence of a malfunction signal. The amount of valve closing is proportional to the malfunction so that the net result will be a valve closing which tracks the malfunction.
The rate at which the valves open and close is set in a rate limiter means having an input from the malfunction proportional control through an interface circuit.
The foregoing is but a brief introduction into the objects, advantages and operation of the present invention. Other objects and advantages will become apparent from the following detailed description of the invention and the novel features will be particularly pointed out hereinafter in the claims.